The MTH turnouts are 20"R. Lionel makes 20"R and 27"R FasTrack turnouts. The 27" are close to #5. American Models makes a #5 but their track is code 148, taller rail than the Lionel FasTrack. Fox Valley Models makes #5 turnouts that use the same .138 rail as FasTrack.
My layout has #5, #6 and #8 turnouts. The #5 turnouts are used only in the freight yard. The mainline and passenger yard use only #6 or #8. The turnouts were custom made for me using MTH rail. Everything I own will work on 30" radius track. The challenge I have with the 80' passenger cars is in a few places we compromised on the length of the easements to make the track plan work. If we had not shortened the easement length all the 30"R curves would be fine.